From my understanding, flock is truely a syscall to the OS you are running the application on. So, in *nix, for example, if you man flock, you will get the manual for this specific function call.

One thing I've always done, therefore avoiding any direct system calls, is create a 'lock file'. Creating a simple file, where nothing is allowed to do 'anything' until the file is removed.

For example:

sleep( 0.001 ) while ( -r '.lock' );

# You made it here, the file is gone
[download]

Naturally, it's always easy to say something like this, however you may be stuck with using flock. In that case, I would research the man pages of the OS you are running to find out the exact behavior of the syscall you're using.